1. Field of the Invention
The present invention relates to vehicle braking systems and more particularly to an improved master cylinder arrangement for such braking systems.
2. Description of the Related Art
Vehicle hydraulic braking systems and the master cylinder designs utilized therein are old and well known and the in a simple form has a fluid reservoir located above a hydraulic chamber which is closed when one end by a spring biased movable piston moves past a port to pressurize fluid in the chamber. The master cylinder functions, upon mechanical actuation (often vacuum boosted) of the movable piston to supply brake applying fluid pressure to one or more vehicle wheel brake actuators. Relatively simple master cylinders are disclosed in U.S. Pat. Nos. 4,320,624 and 6,658,844. It is not uncommon to split vehicle hydraulic circuits into two or more nearly autonomous circuits with one circuit for actuating the front wheel brakes another circuit for actuating the rear wheel brakes. One way to effect such a split is to employ a single master cylinder of the tandem type having two separate, but contemporaneously actuable hydraulic chambers with separate fluid reservoirs or a single shared reservoir. Master cylinders having two separate chambers for two separate braking circuits are known as illustrated in many U.S. Patents including U.S. Pat. Nos. 4,122,596; 5,513,492 and 5,943,863.
The master cylinder disclosed in U.S. Pat. No. 5,943,863 has a housing with a bore therein connected by a radial port (for the primary circuit) and an axial port (for the secondary circuit) to a reservoir and to the brake system through first and second outlet ports. First and second pistons located in the bore are separated by a first return spring to define a first or primary chamber while the second piston is separated from the bottom of the bore by a second return spring to define a secondary chamber. The primary chamber is connected to the radial port through an axial passage in the second piston while the secondary chamber is directly connected to the axial port in the housing to communicate fluid between the reservoir and the bore. The first and second pistons respond to an input force by initially moving within the bore to compress the first and second return springs and allow the heads of linkage members or rods to engage and close normally open poppet valves to seal the axial passage to the radial port and the axial port in the housing thereby terminating communication between the bore and the reservoir. This initial movement for closing the reservoir poppet valves and start building pressure in the master cylinder produces no braking force and is known as dead stroke. More precisely, dead stroke (also called compensation loss) is the amount of primary piston travel required to isolate the reservoir low pressure circuit from the master cylinder high pressure circuit. The desirability of reducing this non-productive dead stroke has been recognized in several recent patents. Rather than axially located poppet valves, U.S. Pat. Nos. 6,978,614 B2 and 7,040,093 employ a pair of axially spaced apart cylinder sidewall seals which span a reservoir inlet port for each piston. Each piston has sidewall apertures which, when located intermediate the two seals, allow fluid flow from the reservoir into the respective pressure chambers.
U.S. Pat. No. 6,978,614 teaches one approach to reducing the dead stroke by employing a master cylinder having a bore in which axial primary and secondary cup-shaped pistons are slidingly fitted, and in which at least one sealing element is disposed in contact with each piston, in defining a supply chamber and a pressure chamber. Each piston comprises a bore, open in the direction of the front pressure chamber, and the secondary circuit piston comprises a valve-forming through-hole, opening into its periphery and into its bore, in order to isolate the front pressure chamber when it passes beyond the annular sidewall seal so as to produce a braking pressure. The front (secondary) pressure chamber associated with at least one piston comprises at least one tubular cylindrical element extending axially outside the piston and rearwards of the annular sidewall seal for the obturation of the through-hole thereby providing a reduced dead stroke. The annular element is interposed in order to seal off the radial valve before the latter passes the seal, U.S. Pat. No. 7,040,093 suggests several different sidewall aperture patterns to achieve such a desired result.
It is highly desirable to improve braking performance by minimizing the dead stroke and achieve such minimization in a manner easily adapted to current master cylinder designs.